Polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), generally referred to as "polyalkylene terephthalates", are common commercial polyesters. Recently, poly(trimethylene terephthalate), (PTT), also called polypropylene terephthalate, has achieved commercial importance because of its elasticity, as measured by its elastic recovery and resilience. Based on the numbers of carbon atoms in the glycol used, the above PET, PBT and PTT are also referred to as 2GT, 4GT and 3GT, respectively.
Polyalkylene terephthalates commonly are produced by one of two routes: (1) by transesterification of a dialkyl terephthalate diester, typically dimethyl terephthalate, with a glycol to form an intermediate bis-glycolate terephthalate, followed by polycondensation to form the polyalkylene terephthalate; or (2) by direct esterification of terephthalic acid (TPA) with a glycol to form a bis-glycolate terephthalate, followed by polycondensation to form the polyalkylene terephthalate.
In producing polyalkylene terephthalates by direct esterification, terephthalic acid and an alkylene glycol are reacted in the presence of a catalyst to form a monomer and water. The water is removed as formed during the reaction. Oligomers having a degree of polymerization of about 4 or less can also be formed. Generally, during an esterification a mixture of monomer and oligomer is produced. This mixture, also referred to as a prepolymer, can then be polycondensed or polymerized at higher temperatures under reduced pressure in the presence of a polycondensation catalyst to form a desired polyester resin that is suitable for carpets, textiles, films and many other end-uses.
These reactions can be carried out in a batch or continuous process. The same or different catalysts can be used for the esterification and polycondensation steps.
Esterification catalysts known in the art include titanium, tin and zirconium compounds. Organo titanium and organo zirconium compounds are disclosed in U.S. Pat. No. 3,056,818 for use as esterification catalysts. The combination of organo tin and organo titanium compounds for the esterification of terephthalic acid and 1,4-butanediol is disclosed in U.S. Pat. No. 3,936,421. The use of tin-titanium complexes as esterification catalysts for 2GT and 4GT is disclosed in U.S. Pat. No. 4,018,708 and U.S. Pat. No. 4,020,010. U.S. Pat. No. 5,015,759 (DuPont) discloses a process for faster direct esterification of a diacid to make 2GT or 4GT using relatively high amounts of an organo titanium, organo tin or organo zirconium catalyst. None of these references discloses or suggest that any of these catalysts can be used to produce 3GT.
The use of 3GT is handicapped by various difficulties in its preparation. Surprisingly, using direct analogs of the processes developed for preparation of 2GT and 4GT do not necessarily give 3GT with satisfactory properties.
For example, relatively high temperature (about 290.degree. C.) esterification is considered commercially acceptable for 2GT made from TPA. However, esterification to produce 3GT under similar process conditions appeared to result in the significant liberation of undesirable by-products, including acrolein and allyl alcohol. In addition, the intermediate 3GT prepolymer was found to be highly discolored under these conditions, an indication of poor 3GT polymer quality. Similar esterification difficulties in processes for the production of 4GT prepolymer by direct esterification have led to a preference for the transesterification route using dimethylterephthalate instead of terephthalic acid. For 3GT, because of the greater availability of terephthalic acid in many countries, it is important to develop a low temperature esterification process for the commercial production of good quality 3GT prepolymer.
U.S. Pat. No. 4,611,049 discloses a process for producing 3GT or 4GT using a sulfonic acid promoter to increase the rate of polymerization when using an organo titanium or organo tin catalyst.
U.S. Pat. No. 5,340,909 discloses the use of an effective catalytic amount of tin for the polycondensation step to make 3GT, wherein about 100 to 650 ppm of tin based on the terephthalic acid is given as the permissible range. To mask the resulting polymer yellowness, a blue pigment may be added prior to the polycondensation step. When the prepolymer is made by direct esterification, a titanium catalyst (0-125 ppm) or a portion of the above tin catalyst (0-650 ppm) may be used during this step. No examples show the use or benefit of either titanium or tin catalysts or both for direct esterification.
In the above processes for 3GT, too high an amount of catalyst results in a color problem, while too low an amount results in an unacceptably slow reaction. In particular, using a high concentration of tin catalyst is inadvisable since it causes discoloration and degradation of polymer as well as the formation of large amounts of undesirable by-products. In addition, a high amount of tin compounds remaining in the final polymer may be undesirable in certain end-use applications. None of the above references specifically disclose a combination of tin and titanium catalysts for the direct esterification of terephthalic acid with 1,3-propylene glycol, nor is there any information to suggest that there would be any advantage in using a combination of these two catalysts for this process.
There is a need for an improved process for the direct esterification of an acid such as, for example, terephthalic acid with 1,3-propylene glycol. There is also a need to reduce the reaction time for esterification, carry out the esterification at relatively lower temperatures, reduce the concentration of tin in the resulting polymer, and produce a product with improved color without the need of a masking pigment.